The chimney is a comparatively recent invention. Prior to the 14th century, man was content to let the smoke from his fire escape into the air through a simple hole in the roof. Chimneys did appear in Europe after the 14th century and in England, they were made of brick. The masonry chimney was the standard for nearly 7 centuries and is still widely used.
As central heating, using gas or oil furnaces became popular and masonry chimneys became more expensive, metal chimneys appeared about 30 years ago. Later, double wall metal chimneys were introduced and they were quite adequate for the generally lower temperature of oil or gas furnaces and open fireplaces burning solid fuels.
During the mid-seventies, however, there was a resurgence in the use of fireplaces, and metal stoves which burned wood and/or coal. Since solid fuels can create higher temperatures, and there is less regulation of the amount of fuel packed into a fireplace or stove, the need for chimneys, both masonry and metal, to withstand higher sustained temperatures was required. Further, "airtight" stoves, that is, those that will suffocate a fire by closing the air intakes, exacerbated the problem of creosote accumulation in the inner wall of the chimney and hence very high temperature creosote fires.
To meet the problem of higher temperatures, the makers of metal chimneys developed the reverse syphon chimney. This chimney had a triple wall with cold outside atmospheric air being introduced from the top of the chimney into the outer chamber which then was drawn down to the base of the chimney and rose up the inner chamber next to the inner flue. While this system keeps the outer wall of the chimney cool so that it does keep combustible material next to the chimney below 200.degree. F. most of the time, it has been found that it keeps the inner flue too cool, thus condensing creosote on the inner flue. If the creosote is permitted to build-up and an overly hot fire ignites the creosote, an extremely hot fire results inside the chimney which can cause damage to the metal chimney by buckling due to the thermal expansion of the metal. Further, the intense heat can cause breaking of welds, and expansion of seams. This structural failure of the inner flue can permit heat to escape to the outer chamber where the increase in temperature can cause ignition of the combustible material surrounding the chimney resulting in a house fire.
Other metal chimney manufacturers have provided a double walled chimney and filled the chamber with a fine powdered insulation. While this keeps the inner flue temperature relatively high and minimizes the creosote build-up problem, extremely hot fires for extended periods of time cause excessively hot flue temperatures, thermal expansion of the metal, buckling of the inner flue and flow of the insulation into buckled areas or out through buckled seams. If air voids form in the insulated chimney wall as the result of very hot fires, "hot spots" occur in the chimney which can ignite combustibles next to the chimney.
It was observed that simply adding additional insulation around the flue liner to keep the outer wall cool even with triple wall chimneys had the adverse effect of permitting the inner flue liner to get too hot resulting in excessive thermal elongation and diameter expansion resulting in seam failure and buckling.
Finally, the tensile strength of steel decreases rapidly above 800.degree. F. Since the thermal expansion of steel increases with increase in temperatures, extremely hot fires can stress the metal chimney beyond the yield point so that it does not return to its original shape. This can open up spaces in the chimney possibly resulting in a fire in the structure at a later time.
In 1954, Leffler, U.S. Pat. No. 2,687,127 was granted a patent on a triple wall metal chimney. No insulation material was used. An unrestricted amount of air was withdrawn from inside the building for cooling the chimney. The inner flue would be excessively chilled, creating a build-up of creosote in the inner chimney flue.
In 1957, Field, U.S. Pat. No. 2,818,060 was granted a patent on a chimney and stove system which included a triple wall uninsulated chimney in which air was drawn from outside the building and down an outer annular passage of the chimney to the fire box of the stove where it served as combustion air. The system, like the thermal syphon chimney, would cool the inner flue and result in the build-up of creosote.